A digital subscriber line (DSL) is a high-speed transmission technology in which data transmission is performed by using a telephone line, that is, an unshielded twisted pair (UTP).
In a DSL system, a system providing multiple DSL accesses is referred to as a DSL access multiplexer (DSLAM). Because of the electromagnetic induction principle, mutual interference is generated among multiple signals sent to a same DSLAM, and is referred to as crosstalk
As shown in FIG. 1, a port 1 of a DSLAM1 is connected to a user terminal 1 (hereinafter referred to as “terminal 1”), a port 2 of the DSLAM1 is connected to a terminal 2, a port 3 of a DSLAM2 is connected to a terminal 3, and a port 4 of the DSLAM2 is connected to a terminal 4. On a DSLAM1 side, a signal sent to the terminal 1 through the port 1 is received by the port 2, and therefore, causes interference to a signal sent to the port 2 by the terminal 2; a signal sent to the terminal 2 through the port 2 is received by the port 1, and therefore, causes interference to a signal sent to the port 1 by the terminal 1 (a case on a terminal side is similar). This interference is referred to as near end crosstalk (Near End crossTalk, NEXT). A signal sent to the terminal 3 by the DSLAM2 through the port 3 is received by the terminal 4, and therefore, causes interference to a signal sent through the port 4 to the terminal 4; a signal sent to the terminal 4 by the DSLAM2 through the port 4 is received by the terminal 3, and therefore, causes interference to a signal sent through the port 3 to the terminal 3 (a case on the terminal side is similar). This interference is referred to as far end crosstalk (FEXT). Further, FEXT is classified into uplink FEXT and downlink FEXT. FEXT received by a terminal is downlink FEXT, and FEXT received by a DSLAM is uplink FEXT.
As the DSL technology evolves, a spectrum is extended to 250 MHz in a next-generation copper bandwidth access technology G.fast, which is far greater than that in a conventional DSL system. Therefore, in a G.fast system, FEXT is far higher than that in the conventional DSL system. For a G.fast system in which one bundle includes 15 cables, when each port is activated and no crosstalk is canceled, FEXT among the cables is nearly 40 dB higher than that when a single port is activated, and a crosstalk amount of the FEXT increases with an increase in frequency.
In order to decrease FEXT among cables in the G.fast system, a linear precoding technology such as a vectored-DSL technology may be used to perform joint transmission and joint reception at the DSLAM end. However, it is difficult to eliminate high-frequency FEXT in the G.fast system by using the linear precoding technology. Therefore, a nonlinear precoding technology based on, for example, QR decomposition (Tomlinson-Harashima-Precoding, QR-THP), appears.
In a G.fast system using nonlinear precoding such as QR-THP, a DSLAM includes a vectoring control entity (VCE) and a G.fast transceiver unit at the side of the operator end of the loop (G.fast transceiver Unit at the side of the operator end of the loop, FTU-O); and a device on a terminal side includes a G.fast transceiver unit at the side of the subscriber end of the loop (G.fast transceiver Unit at the side of the subscriber end of the loop, FTU-R).
In the G.fast system, the VCE is used to perform crosstalk precoding on a transmit signal sent to a terminal by a DSLAM (hereinafter referred to as “downlink”), and is further used to perform crosstalk cancellation on a received signal sent to a DSLAM by a terminal (hereinafter referred to as “uplink”). The FTU-O is used to transmit uplink and downlink data streams and uplink and downlink control messages between the VCE and the FTU-R. Each port corresponds to one FTU-O and one FTU-R, and the FTU-O is connected to the FTU-R by using a line.
In the downlink direction, after a downlink data stream undergoes nonlinear precoding and passes through a crosstalk channel, FEXT still exists when the downlink data stream enters an input end of the FTU-R, and in some scenarios, signal energy of the FEXT may be about 3 dB higher than signal energy obtained when a single port is activated. In the uplink direction, an uplink data stream passes through a crosstalk channel and undergoes crosstalk cancellation, FEXT also exists when the uplink data stream enters an input end of the FTU-O.
As shown in FIG. 2, in the G.fast system, regardless of the uplink direction or the downlink direction, a transmit end sends a signal to a receive end through a channel, and a programmable gain amplifier (PGA) of the receive end adjusts a receive power under control of a PGA training module.
First, in a DSL handshake or initialization process, the PGA training module obtains a target input power Pt of an analog to digital converter (ADC) according to a maximum input power allowed by the ADC and a peak-to-average ratio (PAR) of a received signal of the receive end, where a unit is dB:Pt=ADC maximum power−PAR  formula 1
Then, the PGA training module determines an actual input power P1 of the ADC, and further calculates a power adjustment amount delta=Pt−P1.
Finally, an adjusted PGA gain=current PGA gain+delta is obtained.
After a showtime is entered, that is, after a service is actually run, the PGA gain is unchanged, that is, receive power adjustment is no longer performed.
In the foregoing receive power adjustment process, during calculation of the target input power of the ADC, impact of FEXT is not considered. Therefore, when FEXT exists, the actual input power of the ADC may be excessively large. Consequently, signal clipping may occur in subsequent processing processes of filtering based on down-sampling and Fast Fourier Transform (FFT) of the receive end.
To sum up, in the existing G.fast system, when nonlinear precoding is used to perform FEXT cancellation, impact of FEXT on an ADC input signal of a receive end is not considered, and consequently, the receive end cannot work normally.